The invention relates in general to electrode arrangements for transducers and in particular to a new and useful transducer for the combined measurement of various physiological quantities or parameters, which has a measuring head for recording a first physiological quantity and means for measuring a second physiological quantity. These means comprises a first measuring electrode located on one side of a ring body adjacent the measuring head, and a second counter-electrode on another side of the ring body spaced away from the measuring head.
Such an electrode arrangement for a transducer is known from European Patent application No. 0077054. In this transducer, besides the electrode arrangement for recording a first physiological quantity, which is provided in a measuring head body, an additional electrode arrangement is provided which comprises a first electrode on one side of the measuring head body facing toward a contact area, and a second electrode on the back or opposite side of the measuring body. With such a transducer, the transcutaneous oxygen partial pressure is measured, for example, via the first electrode arrangement and the additional electrode arrangement measures a second physiological quantity such as the cardiac action potential (EKG). The first electrode on the side of the measuring head body toward the contact area serves as the measuring electrode, and the second electrode on the back of the measuring head body as counter-electrode.
In the known electrode arrangement, an adhesive is applied on the bearing surface of the measuring head body provided for that purpose, the measuring electrode being surrounded by this adhesive. Such gluing is to establish an electrical insulation of the measuring electrode from the counter-electrode. However, only a small outer region of the total bearing surface is available for this bonding, so that its durability is limited. Durability is diminished further by the fact that body fluids can penetrate into the adhesive layer, thus reducing the adhesion force of the adhesive layer.
In order to utilize adhesive areas next to the electrode surfaces which are to be connected to the skin, two apparently contradictory requirements must be fulfilled. For one thing, the surfaces of the electrodes should be as large as possible so as to present as low an impedance as possible toward the skin, and hence not to be susceptible to trouble or interference. In addition the adhesive areas of the transducers should be as large as possible, so that in practical use they cannot move away from the contact area even under relatively great tensile and shearing stress. On the other hand, the total area of the adhesive area plus the electrode surface should be as small as possible, so that the transducer can be applied to body parts having small surfaces such as the part of the exposed head of an unborn child (so called fetal monitor). Generally, electrodes for recording physiological quantities are provided with an ion conducting gel for contacting with the skin. The adhesion force of such gels, however, are not sufficient for relatively great loads. When used with comparatively bulky and heavy transducers for transcutaneous oxygen measurement, such gels do not ensure reliable adhesion conditions on the contact area under the complicated conditions occurring in practice.
In a transducer which is suitable for measuring physiological quantities, the adhesive strength is further reduced because a recess is needed for the electrode arrangements in the adhesive area.